The invention disclosed herein relates generally to the field of printing systems and, more particularly to a system capable of imaging specialized lithographic plates for use in printing operations and, more particularly to methods of automatically adjusting such systems for optimum performance.
Commonly a printing operation, undertaken on a printing press, utilizes a lithographic plate which is produced in a separate process involving an exposure of an image onto the plate which is typically a thin aluminium alloy or polyester base suitably treated so as to be sensitive to light or heat radiation.
The method of making a lithographic plate suitable for use on a printing press has commonly been to produce a film mask using a low power laser printer known as an xe2x80x9cimagesetterxe2x80x9d exposing a highly sensitive film media. The film media is usually processed in some manner and then placed in area contact with a photosensitive plate and a flood or area exposure is made through the film mask. The most common lithographic plates used in such a process are sensitive to radiation in the ultraviolet region of the spectrum. The plate is then processed through a chemical solution to develop the image.
More recently, a method of exposing a lithographic printing plate directly through use of a specialized printer known as a platesetter is gaining popularity. A platesetter in combination with a computer system that receives and conditions the image data for sending to the platesetter is commonly known as a Computer-to-Plate or CTP system. CTP systems offer a substantial advantage over imagesetters in that they eliminate the film and the associated process variation associated with that extra step.
The CTP system receives the image data and formats it to make it suitable for outputting to an exposure head which in turn controls a radiation source, which could be a laser, so as to image picture elements (pixels) on the plate. The radiation beams induce a physical or chemical change to the coating on the lithographic plate. It is usually necessary to amplify the difference between the exposed and un-exposed areas in a further chemical processing step that removes the unwanted coating and converts the plate into a lithographic printing surface ready for use on the press. The processing step may include pre-baking the plate in an oven, washing in chemical solutions and possibly a post-processing bake cycle.
In contrast, some lithographic plates now available, do not require a further processing step and the imaging is either fully completed by the platesetter or will be completed once mounted on the press. The typical methods of on-press developing include allowing the non-image areas simply to wear off during the print job make-ready or dissolving these regions in a fountain solution, which is already a standard component in offset printing, or a combination of both methods. Such plates are referred to as xe2x80x9cprocesslessxe2x80x9d since they eliminate the need for a separate processing step although in practice the processing is accomplished by simply running the press. While this step may require quite a number of impressions to sufficiently process the plate, it can be done substantially in parallel with the normal set up of a printing job on the press.
Other processless plates work in an ablative fashion where the unwanted areas are removed by the imaging process. In this case, the imaging and processing are achieved simultaneously although it is usually necessary to provide a debris collection system to draw the ablated products away from the plate surface.
It is quite common for manufacturers of printing presses to include quality monitoring systems on their presses which function to monitor some aspect of the printing process. In some cases, the monitoring system will provide feedback to the press operator or even a closed-loop feedback to a process controller that then effects a correction to the process. There are numerous disclosures of systems that use photosensors, Charge Coupled Device (CCD) cameras or other means to monitor a variety of features of the printed article. Systems capable of controlling the ink feed to the press, controlling print quality and looking for defects in the printed product have all been previously disclosed. Typically such systems reside on the press and either compare the readings of the print density of the printed product to a previously stored reference or they print supplemental test targets of a pre-determined pattern which can then be viewed by the sensor and analysed in order to determine the necessary process corrections.
All of these systems used to monitor the printing process have a common theme of improving the process control of the printing operation. They are either concerned with ensuring that subsequent impressions of an image are of consistent quality or they teach methods of closed loop process control which try to maintain the print quality at some pre-determined target level.
In contrast, the art is not as well advanced in the area of ensuring that the actual plate made in the CTP system or by other means is optimally imaged and processed. For most CTP lithographic plates, particularly those intended for long run printing, the imaging and processing of the plate is critical to ensure that the image areas do not wear off the plate prematurely. Since the CTP system writes the plate directly, it becomes feasible to add monitoring and process control, which can be applied towards controlling the quality of plates. While not impossible to add similar controls to an imagesetter the extra imaging step makes it impractical since it may not be possible to determine whether a particular feature is caused by the imagesetter, the film processing, the contact exposure, or the plate processing. With two of these aforementioned steps removed, the viability of providing good platemaking process control is significantly improved.
In platemaking there are a number of imaging process parameters that need to be optimally set for best results. One of the most critical parameters is the level of exposure given to the plate in the imaging step. Exposure is defined as the amount of radiant energy per unit area that impinges on the plate during the imaging process. Depending on the plate type being imaged it may be necessary to control this parameter within a few percent.
The situation is also further complicated in multi-beam platesetters in that each beam needs to impart a substantially equal exposure to the plate so that imaging errors or artefacts are not created. Unless it can be guaranteed that all beams of a multi-beam exposure head have identical size and propagation characteristics, it is not possible to do a simple power or intensity balance because exposure has both a spatial component and a power or intensity component. While it is possible to directly measure beam size the measurement is quite complicated and accurate results are difficult to achieve. Systems exist which are well suited to beam analysis but they are usually in the form of stand alone equipment and not at all suitable or cost-effective for inclusion in a CTP system.
The pragmatic approach, which is commonly adopted, is to let the plate be the measurement tool. Since the human eye is extremely sensitive to slight variations in a regular pattern a trained operator can make quite an effective diagnosis of an imaged and processed plate and perform the required adjustments to the platesetter based on their observations. Platemakers also make use of a densitometer, which is an instrument that determines the optical density of a substance by measuring the intensity of the light it reflects or transmits. Densitometers are useful for quantifying the ratio of image to non-image area and can assist in determining the correct exposure.
Whether using a densitometer or simply judging plates by eye the process remains manual and requires intervention of trained personnel. As the use of CTP systems grow in popularity, techniques that can be employed to conveniently set up the platesetter and processor combination for optimum performance become increasingly important. Additionally the possibility to exploit the computerization and integration of the CTP platemaking system to enhance process control is clearly a feature that would have significant impact on the usefulness of these systems to the printer.
A method and apparatus are provided for determining the optimal adjustment of an imaging system used to make lithographic plates for a printing press. A series of test patterns are imaged on a lithographic printing plate, while varying a particular imaging parameter. For plate types that require further processing to convert the imaged plate into a surface ready for use on a printing press, the imaging is followed by a processing step. Finally, the plate is read-back on the imaging system where the reflectivity of the imaged test patterns is measured to determine the optimal setting for the imaging parameter.